He explains that adding "Noindex: /" to your robots.txt file will now deindex your complete site. Specifically, Google has told us about the new REP META tags protocol and the X-Robots support a while back, so just be careful with your old tags.

Good catch, Sebastian. How is your experiment going? At the moment we will usually accept the “noindex” directive in the robots.txt, but we are not yet at a point where we are willing to set it into stone and announce full support.